Although the rate of out-of-wedlock births among teenagers ages 15 to 17 has fallen by more than half since 1991, they still add up to nearly 90,000 births per year, and a report from the Centers for Disease Control and Prevention attributes at least part of this stubbornly large number of teen pregnancies to simple sexual ignorance. According to the CDC survey, more than 80 percent of teens in that age group have had no formal sexual education when they first begin engaging in sexual intercourse. Moreover, only 25 percent of teens ages 15 to 17 have even discussed sex with their parents. The report notes that as a consequence, when these teens do use birth control, it is usually one of the least effective kinds, such as the rhythm method.